Xylan is one of the polysaccharides widely existing in nature, and is classified as hemicellulose among polysaccharides, which are the main constituents of cell walls and peripheral tissues of higher plants, such as cellulose, lignin, hemicelluloses, and pectins. The structure is high-molecular-weight polysaccharide having a main chain polymerized by a .beta.-1,4-xyloside bond with xylose being a unit. In nature, xylan exists not only as homoxylan whose constituent sugar is only xylose but also as heteroxylan such as arabinoxylan in which arabinose branches and is bound to the main chain.
Xylanase is the general name for enzymes catalyzing the hydrolysis of xylan via xylosaccharides and xylobiose finally into xylose, and is roughly categorized into endoxylanase, exoxylanase, and .beta.-xylosidase depending on the action modes. Detailed comparison of these action modes establishes a large kind of enzymes. Therefore, it has been thought that xylan in plant tissues is degraded by a synergistic action of xylanases having a variety of action modes.
Xylanase is utilized for production of xylooligosaccharides or xylose from xylan or a treatment of a biomass. In addition, recently, application of xylanase is progressing in the field of enzymes for feeds and food processing, and various kinds of xylanase are under research and development.
A mold Acremonium cellulolyticus has a property that it produces a cellulase having strong saccharification, and usefulness for feed and silage is reported (See e.g. Japanese Patent Laid-open Pub. Nos. Hei 4-117244 and Hei 7-236431). Cellulase component contained therein is also reported (See e.g. Agric. Biol. Chem. 52, 2493-2501 (1988); ibid. 53, 3359-3360 (1989); ibid. 54, 309-317 (1990)). In addition, the crude enzyme produced by said mold produces xylanase together with cellulase, and some components of the xylanase were reported (See e.g. Agric. Biol. Chem. 51, 65-74 (1987); ibid. 51, 3207-3213 (1987); Report of National Institute of Bioscience and Human-Technology, 1, 37-44 (1993), Japanese Patent Pub. No. Hei 1-21957; and Japanese Patent Laid-open Pub. Nos. Hei 1-171484 and Hei 1-171485).
These reports relate to thermostable xylanase and xylooligosyl transferase, and mesophilic xylanase was not known in detail.
It is preferable to use mesophilic xylanase having an activity from neutral to acidic region and in the mesophilic region, the optimal temperature being up to about 60.degree. C. when it is applied for food or feed. Such mesophilic xylanase, however, has not been known so far.